question stringlengths 37 38.8k | group_id int64 0 74.5k |
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<p>I'm trying to set up the problem of deriving the thermodynamics of hadronic matter. I know how to proceed in the case of an effective description such as mean field (<a href="http://arxiv.org/abs/nucl-th/9701058" rel="nofollow">Walecka/linear sigma model</a>) but I'm trying to start from QCD. I do not wish to actual... | 2,813 |
<blockquote>
<p><strong>Question:</strong>
A police boat is chasing a boat with criminals along a straight river
by moving against the stream. The speed of the river stream is 3 miles
per hour, the speed of the boat with criminals relative to the river
is 30 miles per hour, and the police boat is 4 miles per ... | 2,814 |
<p>If I have a long solenoid, e.g. length $l$ and radius $r$ with $l = kr$, where k >> 1, with a nonpermeable (e.g. air) core, how much of the magnetic energy is stored outside as compared to inside?</p>
<p>If I go by the Wikipedia article on <a href="http://en.wikipedia.org/wiki/Solenoid" rel="nofollow">solenoids</a>... | 2,815 |
<p>suppose there is a scale able to measure weight with an uncertainty of $10^{-9}kg$ . On the scale, an airtight plastic chamber is placed. Initially, a fly of mass $10^{-5}kg$ is sitting at the bottom of the chamber, which sits on the scale. At a later point in time the fly is flying around the chamber. Will there be... | 792 |
<p>I believe in Neutron Scattering the neutrons after hitting a nucleus can bounce in any of 360*3 dimensions -> 1080 degrees?</p>
<p>Why is this so? Shouldn't it only bounce "off" the neutron in approximately the same "direction" that it came in such as when a particle bounces off a mirror -> because of the cross-sec... | 2,816 |
<p>I would like to ask what are the experimental evidences that led to the conclusion that QCD is the right theory to describe strong interactions. I know that some of the key point are the decay of $\pi_{0}$ and the measurement of Jets but I'd love to see a full answer to this question. Is there a still a chance to <a... | 2,817 |
<p>I have been wondering recently how useful programming is to a physicist. It seems fairly useful (simulations are a lot cheaper than the actual thing in many cases) in some areas (say space programs), but in other areas (for example at CERN) it seems pointless. So is it useful and perhaps how do you use it?</p> | 2,818 |
<p>For me, the gradient of a scalar field (say, in three dimensions) is simply (formally)</p>
<p>$\nabla f = \left(\frac{\partial f}{\partial x}, \frac{\partial f}{\partial y},\frac{\partial f}{\partial z} \right)$.</p>
<p>In which way do we need a metric?</p>
<p>But some people did tell me only on a Riemann manifo... | 2,819 |
<blockquote>
<p>The ball of mass $m$ is given a speed of $v_a = \sqrt{3gr}$ at position $A$. When it reaches $B$, the cord hits the small peg $P$ after which the ball describes a smaller circular path. Determine the position $x$ of $P$ so that the ball will be able to reach point $C$.</p>
</blockquote>
<p><img src="... | 2,820 |
<p>Maybe this is a silly question, but why does, say, a gasoline-powered AC generator have to use more gas depending on the load?</p>
<p>Let's say I have a 120VAC generator and either a 1A or a 10A load, and assume it can handle 1200W without issue. For 10A, the generator uses more gas and works harder compared to 1A.... | 2,821 |
<p>Law of equipartition predicts the heat capacity of gases correctly. It assumes that inter-molecular attraction in gases is negligible (which is true). But for solids, inter-molecular attraction is not negligible, the, how come it still predicts the correct value for molar heat capacity?</p>
<p>How can we ignore the... | 2,822 |
<p>As part of a hw problem for a class, we're supposed to be deriving the equivalence given in equation 2.3 of this paper <a href="http://arxiv.org/abs/1107.5563" rel="nofollow">http://arxiv.org/abs/1107.5563</a>. I was wondering if there is some special relation involving the Ricci Curvature in 5d's relationship to o... | 2,823 |
<p>I am doing a (mostly qualitative) course on Particle Physics, and am confused about the concept of <a href="http://en.wikipedia.org/wiki/Asymptotic_freedom" rel="nofollow">asymptotic freedom</a>. The lecture notes basically say that a quark may experience no force/be "unbound" temporarily as a result of a collision.... | 2,824 |
<p>I'm trying to find the potential energy of multiple geometric shapes made entirely out of point charges.
This particular shape is a cube made out of two different point charges, A and B, each separated by a constant distance.</p>
<p>I'm not entirely sure how to go about this problem but I'm pretty sure that the equ... | 2,825 |
<p>I understand that the electromagnetic spectrum is made up of different frequencies of light waves, but is this true in all cases such as with longer wave frequencies? "such as with microwaves". sometimes I get the impression that with microwave ovens for example use waves of electrons, so my question is, is there so... | 2,826 |
<p>If I understand correctly, lightning is the discharge of electricity from the atmosphere into the planet. However, if I switch on a lamp, the wires are not causing thunder (or any audible sound).</p>
<p>I've also heard that the thunder comes from lightning breaking the sound barrier. This sounds weird to me since... | 2,827 |
<p>The problem of the infinite resistor grid is very common. The solution for the resistance between any 2 nodes in an infinite resistor lattice is all over the internet.</p>
<p>My question is somewhat similar but more pragmatic.
If we had a grid that was very large but yet finite... Then what would be the average vol... | 2,828 |
<p>Question:
A bungee jumper jumps from a bridge. The length of the loose rope is 30 m. When the jumper reach the lowest point possible, the rope stretches 10 m. What is the final stretch of the rope, when the oscillation of the rope stops? Mechanical energy loss is null. Hook law applies for the elastic.</p>
<p>I thi... | 2,829 |
<p>A system might have internal energy and/or kinetic energy. Kinetic energy in classical mechanics is a form of energy the object has, only because of its relative movement to other objects. </p>
<p>If you have a system like an oscillator, then you have a system where the amount of internal energy (energy of the spri... | 2,830 |
<p>so one uses equations of motion to describe liquids (e.g. Navier–Stokes equations). These are equations for $\vec{v}(\vec{r},t)$ with boundary conditions on the surface $S$ of the liquid (e.g. $\vec{v}(\vec{r}\in S,t) = \vec{0}$).</p>
<p>How should one incorporate surface tension $\sigma$ in these equations/boundar... | 2,831 |
<p>It is simple to show that under the gauge transformation $$\begin{cases}\vec A\to\vec A+\nabla\chi\\
\phi\to\phi-\frac{\partial \chi}{\partial t}\\
\psi\to \psi \exp\left(\frac{iq\chi}{\hbar}\right)\end{cases}$$
The Schrodinger equation $$\left[-\frac{\hbar^2}{2m}\left(\nabla-\frac{iq\vec A}{\hbar}\right)^2+q\phi \r... | 2,832 |
<p>Quantum fields are presented as operator-valued <em>distributions</em>, so that the <em>operators</em> in the theory are linear functionals of some test function space. This works well for free fields, giving us a particular form for VEVs, in which the 2-point connected correlation function determines the whole theo... | 2,833 |
<p>Source:
<a href="http://www.engineeringtoolbox.com/sound-power-intensity-pressure-d_57.html" rel="nofollow">http://www.engineeringtoolbox.com/sound-power-intensity-pressure-d_57.html</a></p>
<p>Both sound intesity and pressure level are measured in dB. Given a specific sound, are these two dB values the same?</p> | 2,834 |
<p>I read that </p>
<p>$(FF(f))(x)=2\pi f(-x)$, where $F$ is the Fourier transform</p>
<p><em>and</em> $F(f(x-a))(k)=\exp(-ika) X(k)$ where $X(k)=F(f(x))$</p>
<p><em>implies</em> $F(\exp(iax)f(x))(k)=X(k-a)$.</p>
<p>But I don't see how that is done... I am quite happy with getting $F^{-1}X(k-a)=\exp(iax)f(x)$ by br... | 2,835 |
<p>Suppose you a have an ordinary Luttinger liquid with<br>
$$ H = \int dx \sum _{\eta= \pm 1 , \sigma =\uparrow,\downarrow } \psi^\dagger_{\eta, \sigma} (x) (-i v \eta \partial _x) \psi _{\eta,\sigma} (x). $$</p>
<p>You then bosonize it using
$$\psi_{\eta,\sigma}=\frac{1}{\sqrt{2 \pi \alpha}} F_{\eta ,\sigma} e^{-i... | 2,836 |
<p>How can it be shown that the Dirac spinor is the direct sum of a right handed Weyl spinor and a left handed Weyl spinor?</p>
<p>EDIT:- Let $\psi_L$ and $\psi_R$ be 2 component left-handed and right-handed Weyl spinors. Their transformation properties are known. When I put these two spinors in a column and construct... | 2,837 |
<p>For two particles, $\langle {\mathcal T} a(t_1) a^\dagger (t_2) \rangle = \langle
a(t_1) a^\dagger (t_2)\rangle \theta (t_1-t_2) + \xi \langle a^\dagger (t_2)a(t_1) \rangle \theta (t_2-t_1)$ with $\xi$ is a plus sign for bosons and a minus sign for fermions.</p>
<p>How would I write, for example, $\langle {\mat... | 2,838 |
<p>Consider that we have two balls, one white and one black, and two distant observers A and B with closed eyes. We give the first ball to the observer A and the second ball to the observer B. The observers don't know the exact color (state) of their balls, they know only the probability of having one or another color,... | 2,839 |
<p>The time is treated differently in special relativity and quantum mechanics. What is the exact difference and why relativistic quantum mechanics (Dirac equation etc.) works?</p> | 2,840 |
<p>Am I correct in assuming it is the weight of all that liquid + gravity?</p> | 2,841 |
<p>By chance(playing around really) I saw that a spring(mainly from a pen) placed on a neodymium hard-disk magnet(and then flicked by your finger at the top) makes a nice-effect (see youtube video ). It appears to oscillate in slow-motion(looks like tornado).</p>
<p>Of course, "slow-motion" is purposely simplistic and... | 2,842 |
<p>Will <a href="http://en.wikipedia.org/wiki/Timbre" rel="nofollow">timbre</a>/quality be different if two different people play the same guitar? Assume that frequency/pitch and amplitude are same.</p> | 2,843 |
<blockquote>
<p>Four rods A, B, C, D of same length and material but of different
radii r, 2r , 3r and 4r respectively are held between two rigid
walls. The temperature of all rods is increased by same amount. If the
rods do not bend, then which of these are correct:</p>
<ol>
<li>The stress in the rod... | 2,844 |
<p>See this related question: <a href="http://physics.stackexchange.com/questions/118927/if-particles-are-excitations-what-are-their-fields">If particles are excitations what are their fields?</a></p>
<p>I ask this question because, according to a lecture, the higgs boson was frozen into a "matrix" at some point befor... | 2,845 |
<p>My home town (Iran, Lamerd) has very hot summer (always > 45+ degree) with dry weather and sharp sun shine. I used some black tubes under glassy box to heat up water. It was a natural thermal siphon and did not need pimping. definite volume of salty water in the container was connected to the tube glassy box. the em... | 2,846 |
<p>When a polished piece of metal (or steel in particular) is heated to incandescence, how do its reflective properties change?</p>
<p>Given a mirror-like surface, would the object temporarily cease to act like a mirror at a certain temperature?
Would the specular reflection of light turn into diffuse reflection; in ... | 2,847 |
<p>According to the definition of potential energy, we use $U= mhg .
$
In the figure below , </p>
<p><strong>A thin uniform rod of mass m and length h is positioned vertically above an anchored frictionless pivot point.</strong></p>
<p>Why does a author say that potential energy is $U =\dfrac{mgh}2$ why not $U =mgh$... | 2,848 |
<p>It is said that in Minkowski spacetime, the current conservation law for the number current $N^\mu$ where $N^0$ is the number density and $N^i, i=1,2,3$ is the particle flux in the $x^i $ direction, is given by </p>
<p>$$\partial_\mu N^\mu=0 ....................(*)$$</p>
<p>What I don't understand is he following... | 2,849 |
<p>I'm investigating the velocity field induced by a continuous distribution of 2D vortex points distributed along an ellipse $\{a\cos\theta,b\sin\theta\}$. I'm interested in the field inside the ellipse, and I need some help to prove whether this field is zero or not.</p>
<p>The intensity of each vortex point is prop... | 2,850 |
<p>Consider a "wavefunction" $\psi(x)$, which has a Fourier transform $\tilde \psi(p)$</p>
<p>Suppose that we know, for each $x$, $|\psi(x)|^2$, and that we know, for each $p$, $|\tilde \psi(p)|^2$.</p>
<p>Have we enough information to reconstruct the "wavefunction" $\psi(x)$, that is, obtain the phase of the "wavefu... | 2,851 |
<p>The PACER project is described in this question: <a href="http://physics.stackexchange.com/questions/21405/how-much-of-the-energy-from-1-megaton-h-bomb-explosion-could-we-capture-to-do-us">How much of the energy from 1 megaton H Bomb explosion could we capture to do useful work?</a></p>
<p>Why was it abandoned? It ... | 2,852 |
<p>The other day I was wondering: When a <a href="http://en.wikipedia.org/wiki/Tachyon" rel="nofollow">tachyon</a> is coming towards you faster than the speed of light, will you see it before it hits you? Then I thought of course not, since the light waves aren't traveling faster than the tachyon then how could you see... | 2,853 |
<p>Given a parallel plate capacitor of width $w$, length $l$, with a dielectric moving along the length $l$. Let the dielectric be from $x$ onwards.</p>
<p>The capacitance will be $\frac{w \epsilon_0}{d} (\epsilon_r l - \chi_e x)$. Griffiths (p. 195) says that the total charge $Q$ in the $C=\frac{Q}{V}$ expression is ... | 2,854 |
<p>I was just wondering... I believe that if a car travelling 50 miles per hour crashes into a wall, the result should be the same as crashing to another car also travelling 50 miles per hour (but in the other direction of course)</p>
<p>Is this true? Why is that?</p> | 58 |
<p>Let's say there is a fin that is 1mm thick, extends 8mm from the surface, and is 10 mm wide. The fin is exposed to a moving fluid. Can we assume the adiabatic tip condition and use the characteristic, "Corrected", length for calculations?</p> | 2,855 |
<p>Can you trigger a thermonuclear explosion from a smaller thermonuclear explosion in a scaling way, so that starting from a small laser ignited fusion within a small fissile container, using the X-rays from the first explosion to implode a tiny adjacent Li-d wire, which is then used to implode a bigger wire, and so o... | 2,856 |
<p>One method of introducing electric field is based on the measurement of the force acting on moving charged particle. By equating F to qE we determine the electric field E if the electric charge q is known. Similarly, one can determine magnetic field. </p>
<p>This approach is fine for vacuum but it generally fails f... | 2,857 |
<p>Please help me with an answer to my dilemma:</p>
<p>Is there a liquid that could be used to fill an ice rink (non-explosive, non-poisonous, etc), and have the freezing point above 0 Celsius?</p> | 2,858 |
<p>A recent paper, titled <a href="http://arxiv.org/abs/1302.2775">Inertia from an Asymmetric Casimir Effect</a>, discusses the universal horizons relative to an accelerating observer (Rindler space). A figure it used to demonstrate its point challenged a view I held.</p>
<p><img src="http://i.stack.imgur.com/BAWzq.p... | 2,859 |
<p>This is the known equation of air drag:</p>
<p>$$m{\bf a}=mg-\mathcal D=mg-b{\bf v}.$$</p>
<p>Considering this, is air drag equation in term of momentum still valid?</p>
<p>$$m{\bf v}=mv_g-b{\bf r}.$$</p> | 2,860 |
<p>I have created a simulation of <strong>one</strong> electron bouncing through a 3D mesh of molecules. The electron hopping is determined by a calculation of electron transfer rate using the Marcus equation (a result in units of $1/s$). In order to force the electron to reach one end of this 3D mesh I have applied a ... | 2,861 |
<p>I am interested in gravitational lensing caused by a cluster of galaxies (say it has a diameter of 1 Mpc and mass of $10^{12}$ solar masses). How close must a light travel as it passes by to be notably bent - if compared to a case where there would be much smaller object in a spot where is now a center of cluster, l... | 2,862 |
<p>I just started learning physics 3 days ago and am having trouble understanding what I am doing wrong. Can someone please explain my error(s)? Thanks!</p>
<p>We have a 1kg object on a plane at a 30 degree angle from horizontal. Force of friction is 1.5N. We are asked to calculate the net force. </p>
<p>I assume the... | 2,863 |
<p>In quantum mechanical systems which have classical counterparts, we can typically recover classical mechanics by letting $\hbar \rightarrow 0$. Is recovering Einstein's field equations (conceptually) that simple in string theory?</p> | 59 |
<p>This is really a basic question whose answer I guess may have to do with the way we construct Feynman rules and diagrams. The question is: Suppose I have been given a two-point function (found in some other ways, say for example some gauge/gravity duality or some symmetry in the theory). How can we construct the Lag... | 2,864 |
<p>Here's an example to describe the issue</p>
<p>Supossed a high power laser (eg a 100 kW laser, ie, electromagnetic weapons)
is fired to a target, then it will receive energy and move.
(and likely to burn or explode, but that's not the point)</p>
<p>My question is, statting that photons have no mass</p>
<p>Would t... | 2,865 |
<p>Suppose there's a radioactive material and a 1/2 quantum probability of detecting it by a Geiger counter. This puts the system in a superposition. Also suppose you are in the same room, and the walls of the room are perfectly decoherence-proof. You observe the Geiger counter and get a definite result. Either it had ... | 2,866 |
<p>Let us consider Bloch wave function solutions for a particle confined to a 2D square lattice with a potential of the form
$V=V(x) + V(y)$
(that is, one that can be factorized).</p>
<p>In this case we can factorize the Hamiltonian to
$\hat{H} = \hat{H}_x + \hat{H}_y$
and the band spectrum structure (the energy dis... | 2,867 |
<p>How many virtual gravitons do the sun and earth exchange in one year?<br>
What are their wavelengths?</p> | 2,868 |
<p>A typical problem in quantum mechanics is to calculate the spectrum that corresponds to a given potential. </p>
<ol>
<li>Is there a one to one correspondence between the potential and its spectrum? </li>
<li>If the answer to the previous question is yes, then given the spectrum, is there a systematic way to calcula... | 2,869 |
<p>A mass spring system is in equilibrium. If I pull on the load by $x$ meters, the energy stored in the spring is (this is what is given in my book):</p>
<p>$$E=\frac12kx^2 $$</p>
<p>However, doesn't the load lose gravitational potential energy as it moves down? Where would this energy go? By conservation law, shoul... | 2,870 |
<p>I was wondering what voltage or current, if any, would be produced if the basic magnet through a copper coil experiment had the poles rotated 90 degrees so north and south faced the top/bottom of the coil rather than the entry/exit points?</p>
<p>The attached image has a basic illustration of this.</p>
<p>Thanks<... | 2,871 |
<p><img src="http://i.stack.imgur.com/crCx4.png" alt="enter image description here"></p>
<p>The spectrum of a filament has been given before, the left one having the lowest temperature, the middle with a medium temperature and the right one with the highest. My question is this: Why does the region of yellow light on ... | 2,872 |
<p>I was thinking about an apparently simple question about quantum mechanics, if I am looking at a quantum system described by a Hilbert space $\cal{H}$ under what hypothesis can I define A and B as subsystems whose union gives the full former system and decompose $\cal{H} = \cal{H_A}\otimes \cal{H_B}$. The other way,... | 2,873 |
<p>What I mean is, the nuclear chain reactions take microseconds for every generation and that is the reason that nuclear weapons exist. Because in nuclear reactors the reaction rate is much slower thus it can be controlled and prevent it from exploding.
So my question here is about the speed of matter-antimatter reac... | 2,874 |
<p>I apologize if I am missing something obvious, but I am in my first class with tensors and I am still learning the notation. I am running into a problem with the transformation of the transformation of the four-tensor for electromagnetism that is given by</p>
<p>$$
F^{\mu \nu} =
\left[
\matrix
{
0 & - \cal{E}_... | 2,875 |
<p>It's my understanding that Earth's core is hot and molten because of the high pressure from gravity compressing the planet's mass towards the center. How does this heat reconcile with thermodynamics, meaning, would it be possible for all the heat to bleed away into space through infrared radiation etc., such that t... | 2,876 |
<p>I've studied something so I'm able to explain better what I'm thinking about.</p>
<p>'t Hooft duality states:</p>
<p>In a gauge theory with fields in the adjoint representation in presence of a mass gap only the following alternatives are allowed:</p>
<p>1 Magnetic condensation and electric confinement</p>
<p>2 ... | 2,877 |
<p>There was a reason why I constantly failed physics at school and university, and that reason was, apart from the fact I was immensely lazy, that I mentally refused to "believe" more advanced stuff until I understand the fundamentals (which I, eventually, never did).</p>
<p>As such, one of the most fundamental thing... | 558 |
<p>Assuming I rotate a disk, I want to know how long it takes to completely stop, and the number of revolutions it made since I removed my fingers off the disk.</p>
<p>Lets say a DVD I rotate with my fingers. I only know the radians per second (velocity) of the last moment I touched the disc.</p>
<p>Can you guys tell... | 2,878 |
<p>I'm trying to understand a particular case of gauge theories, namely discrete spaces on which a group G can act transitively, with a gauge group H which is discrete as well.</p>
<p>From what I've already read about continuous and discrete gauge theory (I can provide the necessary references if needed*), it seems th... | 2,879 |
<p>I am not exactly sure how the physics work here. If we take a car tow rope, which is manufactured for towing a 3500 kg car and we actually start towing such a car on this rope, what force do we really apply to the rope?</p>
<p>Could the same rope <strong>safely</strong> withhold an adult person (100 kg) hanging on ... | 2,880 |
<p>I'm thinking about a project to tackle, and I'd like to make a simulation that allows the user to define a rope or chain of length L, pin it at arbitrary points r1, r2.... etc. and draw the resulting curve in real time.</p>
<p>Also, I'd like the user to be able to alter the field that the rope exists in, for exampl... | 2,881 |
<p>I'm wondering if it would be possible to map out all the different types of molecules, atoms and nuclei and their energy levels on one page (even if in a generalised way)? But perhaps I'm referring to the periodic table here? Do representations differ according to whether one is looking at things from a Classical pe... | 2,882 |
<p>I learned that as the earth rotates about its axis, the bodies on the earth also follow a circular path. In most books I read, they give the example of a person standing on a weight balance at the equator... and I did understand that. However, by doing the following calculation, I am seeing that the apparent weight ... | 2,883 |
<p>In most of the field theory text they will start with lagrangian density for spin 1 and spin 1/2 particles. But i could find any text where this lagrangian density is derived from first principle. </p> | 2,884 |
<p>If you stick a tissue to the bottom of a glass and immerse this glass fully (upside down) into a tumbler of water, then this tissue remains dry. Why is this so? / What conclusion would you draw from this?</p>
<p>a) Air occupies a definite volume.</p>
<p>b) Air has mass.</p>
<p>c) Both</p>
<p>Now, I think it shou... | 2,885 |
<p>I'm trying to imagine in my head what happens to the universe in ten billion years, twenty billion years, etc. </p>
<p>I imagine that all mass in the universe turns to black holes, as when mass reaches the temperature of absolute zero, it automatically collapses the atoms, or is that true?</p>
<p>If all of the mas... | 2,886 |
<p>If a detector is kept at the two slits the fringes disappear. But, when the detectors are removes do the fringes come up immediately without any significant time lag? </p>
<p>Can there be a way to switch on and off the detector so fast that it all happens within the time required for the light to reach the screen f... | 2,887 |
<p>This question seems to have been asked a few times in different configurations, but none of them answer my variation. I've struggled to understand this for nearly 15 years and had conflicting answers from my school physics teachers and more recently friends who are physicists.</p>
<p>So a round leaves my gun barrel... | 2,888 |
<p>So far I'm only tasting the quantum mechanics. Haven't gone very deep into the mathematics of it yet.</p>
<p>I read about the double slit experiment, and the weird consequences of it: if you put a detector at the slits you won't have an interference pattern. Since you basically absorbed and recorded the photon then... | 2,889 |
<p>Question on Section 9.1.3 in "Conformal Field Theory" by Philippe Di Francesco et. al.</p>
<blockquote>
<p><em>The basic idea of the Coulomb-gas formalism is to place a background charge in the system, making the $U(1)$ symmetry anomalous. This has the effect of modifying the conformal dimensions of the vertex op... | 2,890 |
<p>$E = h\nu$ and $P = h\nu/c$ in vacuum.
If a photon enters water, it's frequency $\nu$ doesn't change.
What are its energy and momentum : $h\nu$ ? and $h\nu/c$ ?
Since part of it's energy and momentum have been transferred to water, it should be less.</p>
<p>If water's refractive index is $n$, Are the energy and mom... | 1,006 |
<p>consider the following:
<img src="http://i.stack.imgur.com/tpCcw.jpg" alt="enter image description here"></p>
<p>I need to find the minimal coeeficient of friction $\mu _{min}$ so that both recatngle boxes would remain static. The lower angle in the triangle is $2\alpha$ as indicated.
I ended up with this expressio... | 2,891 |
<p>I have seen a lot of places talking about the Reynolds number and how it is calculated, but I have never seen an equation that actually made use of this number to calculate lift, drag, or other aerodynamic properties.</p>
<p>So, what is this number actually used for?</p> | 2,892 |
<p>I just read that in the Gaussian Units of charge The Final equation in Coulomb's law is as simple as $$\boldsymbol{F}=\frac{q_1q_2}{r^2}$$</p>
<p>No $\epsilon_0$ no $4\pi$ like you have in the $\mbox{SI}$ units of measurement . </p>
<p>The permittivity constant was the factor in the $\mbox{SI}$ system of Coulomb... | 2,893 |
<p>What was the proportion of dark matter/energy to other matters/energy at the moments after the beginning of the universe (standard Big Bang model)?</p> | 2,894 |
<p>When correlation function has branch cut in momentum space,
how to find correlation in coordinate space?
For example
$$ \tilde {G}(\omega) = \frac{2i}{\omega+(\omega^2-\nu^2)^{1/2}}$$
How to get the $G(t)$ usng Fourier transformation ?<br>
t>0 HERE.
This problem is from matrix model of Iizuka and Polchinski. They d... | 2,895 |
<p>The masses of the Z and W particle sum almost exactly to the mass of the Top quark,within the errors:</p>
<p>Z + W = 80.385±0.015 + 91.1876±0.0021 = 171.57 ±0.0171 GeV</p>
<p>Top quark 172.9± 1.5 GeV</p>
<p>A: Is this one of those simple coincidences?</p>
<p>B: The Z,W particles are decays of the T?</p>
<p>C: S... | 2,896 |
<p>So a slackline is basically a bouncy tight rope. </p>
<p>I found a site that has a calculator for the tension of a static slackline </p>
<p><a href="http://www.slacklineexpress.com/force.php?linelength=40&sag=1&w=170" rel="nofollow">http://www.slacklineexpress.com/force.php?linelength=40&sag=1&w=17... | 2,897 |
<p>Consider 2 uncorrelated photon pairs (a1,a2), (b1,b2) such that (a1,a2) are entangled, and separately (b1,b2) are entangled. We wish to entangle-swap so as to end up with a new entanglement (a1,b1) by using the ancillary photon pair (a3,b3), such that (a1,a3) are entangled and (b1,b3) are entangled. This suggests th... | 2,898 |
<p>Why does dust stick to rotating fan propeller?</p>
<p>Intuitively, most people (including I) think of the dust will not stick to rotating fan propellers.</p>
<p><strong>EDIT 1:</strong></p>
<p>Thank you for the great explanations. I am still waiting for the "better" one if any.</p> | 945 |
<p>If the light velocity is a vector quantity, why vector addition cannot be applied to it?</p>
<p>Or the light velocity is not a vector quantity?</p> | 2,899 |
<p>In quantum information and quantum computation, we generally use Bell type states which are maximally entangled. I find that the set of entangled states as interesting objects from a mathematical point of view and one can ask many questions regarding their structure and so on. But my question is, what are the practi... | 2,900 |
<p>A circuit, has current $A$ flowing at a certain $V$.</p>
<p>When there is a change in magnetic-flux, based on Faraday's law of induction & Lenz's law, we know that there is change in <strong>Potential Difference</strong> now, aside from the source $V$ now we have a induced $-V$ due to the change in magnetic-flu... | 2,901 |
<p>I've known for a long time that if you heat a magnet, there is a point it loses its magnetism (the <a href="https://en.wikipedia.org/wiki/Curie_temperature" rel="nofollow">Curie temperature</a>). It isn't clear to me if this applies to induced magnetism like iron sticking to a magnet.</p>
<p>Will molten iron behav... | 2,902 |
<p>In an alternating current, the flow of electric charge periodically reverses direction, and the number of times it does that is called the frequency of the current. However, if the frequency of an AC is made infinite, would any current flow? I asked this question to my school teacher and he was unable to provide me ... | 2,903 |
<p><strong>Space-like</strong> separated events are events that, in a well-chosen reference frame, can take place at the <strong>same time</strong> but <em>never</em> happen at the <em>same</em> <em>location</em>.</p>
<p>On the other hand for <a href="http://en.wikipedia.org/wiki/Spacetime#Time-like_interval" rel="no... | 2,904 |
<p><img src="http://i.stack.imgur.com/in7Xf.jpg" alt="enter image description here"></p>
<p>A sends out a series of flashes of light to B, where the interval between flashes is denoted by T according to A's clock. Then it is plausible to assume that the intervals of reception by B's clock are proportional to T, say K... | 2,905 |
<p>Given that in the ballistic regime a particle (electron) can move freely without scattering (there are no impurities ), is the resistance through a ballistic sample zero?</p> | 2,906 |
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